Hongnian Cai

Plastic Deformation Mechanisms of AZ31 Magnesium Alloy under High Strain Rate Compression

The twinning and slip activities of AZ31 magnesium alloy sheet at a strain rate of 1200 s^(-1) were investigated. Dynamically mechanical properties of various oriented samples were measured using Split Hopkinson Pressure Bar (SHPB). Optical microscope observations reveal that the dominant deformation mechanism is twinning for 90°oriented sample, and is slip for 45° and 0° oriented samples. TEM analysis for samples at a strain of 0.3% shows that the main deformation mechanisms for 90°, 45° and 0° oriented sample are {10(average)12} ＜10(average)11＞ and {10(average)11}＜10(average)12＞ twinning, basal slip and non basal slip, respectively. The main features of the true stress-true strain curves can be explained based on deformation mechanism analysis.

Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy processed by equal channel angular pressing

Abstract Microstructure evolution and texture development and their effects on mechanical properties of a Mg-Gd-Y-Zr alloy during equal channel angular pressing (ECAP) were investigated. It is found that the microstructure is still inhomogeneous after four passes, and two zones, namely the fine grain zone (FGZ) and the coarse grain zone (CGZ) are formed. The grain refinement occurs mainly by particle-stimulated nucleation (PSN) mechanism, which led to a more random texture after four passes of ECAP. In the ECAP-processed alloy, the strength did not increase while the ductility was enhanced dramatically compared with the as-received condition. The change of ductility of this alloy was discussed in terms of texture and second phase particles.

Microstructure characteristics and mechanical properties of TiB/Ti-1.5Fe-2.25Mo composites synthesized in situ using SPS process

Abstract TiB/Ti-1.5Fe-2.25Mo composites were synthesized in situ using the spark plasma sintering (SPS) method at temperatures of 850-1150 °C. The effect of the sintering temperature on microstructure and mechanical properties of the composites was investigated. The results indicate that the aspect ratio of the in situ synthesized TiB whiskers in Ti alloy matrix decreases rapidly with an increase in sintering temperature. However, both the relative density of the sintered specimens and the volume content of TiB whiskers in composites increase with increasing sintering temperature. Thus, the bending strength of the composites synthesized using SPS process increases slowly with increasing the sintering temperature from 850 to 1150 °C. TiB/Ti-1.5Fe-2.25Mo composite synthesized at 1150 °C using SPS method exhibits the highest bending strength of 1596 MPa due to the formation of fine TiB whiskers in Ti alloy matrix and the dense microstructure of the composite.

Adiabatic shear fracture in Ti-6Al-4V alloy

Abstract Separated specimens of Ti-6Al-4V alloy were dynamically loaded at a strain rate of 3 900 s −1 using a split Hopkinson pressure bar (SHPB) apparatus. The fracture features of the separated specimens were investigated by a scanning electron microscope. The results show that adiabatic shear failure occurs in the tested specimens, and two typical areas (dimple and smooth areas) with different features are alternatively distributed on the whole fracture surface. The dimple areas originate from voids generation and coalescence, exhibiting ductile fracture characteristics. Simultaneously, ultrafine grains (UFGs) and microcracks among grains are observed on the smooth areas, indicating that the emergence of UFG areas is caused by the propagation of microcracks along grain boundaries and exhibits brittle fracture characteristics. Fracture occurring in adiabatic shear bands is not uniform and ultimate rupture is resulted from ductile and brittle fracture modes.

Quasi-static and dynamic tensile behaviors in electron beam welded Ti-6Al-4V alloy

Abstract The quasi-static and dynamic tensile behaviors in electron beam welded (EBW) Ti-6Al-4V alloy were investigated at strain rates of 10−3 and 103 s−1, respectively, by materials test system (MTS) and reconstructive Hopkinson bars apparatus. The microstructures of the base metal (BM) and the welded metal (WM) were observed with optical microscope. The fracture characteristics of the BM and WM were characterized with scanning electronic microscope. In Ti-6Al-4V alloy joint, the flow stress of WM is higher than that of BM, while the fracture strain of WM is less than that of BM at strain rates of 103 and 10−3 s−1, respectively. The fracture strain of WM has apparent improvement when the strain rate rises from 10−3 to 103 s−1, while the fracture strain of BM almost has no change. At the same time, the fracture mode of WM alters from brittle to ductile fracture, which causes improvement of the fracture strain of WM.

High-content ductile coherent nanoprecipitates achieve ultrastrong high-entropy alloys

Precipitation-hardening high-entropy alloys (PH-HEAs) with good strength−ductility balances are a promising candidate for advanced structural applications. However, current HEAs emphasize near-equiatomic initial compositions, which limit the increase of intermetallic precipitates that are closely related to the alloy strength. Here we present a strategy to design ultrastrong HEAs with high-content nanoprecipitates by phase separation, which can generate a near-equiatomic matrix in situ while forming strengthening phases, producing a PH-HEA regardless of the initial atomic ratio. Accordingly, we develop a non-equiatomic alloy that utilizes spinodal decomposition to create a low-misfit coherent nanostructure combining a near-equiatomic disordered face-centered-cubic (FCC) matrix with high-content ductile Ni3Al-type ordered nanoprecipitates. We find that this spinodal order–disorder nanostructure contributes to a strength increase of ~1.5 GPa (>560%) relative to the HEA without precipitation, achieving one of the highest tensile strength (1.9 GPa) among all bulk HEAs reported previously while retaining good ductility (>9%).High entropy alloys usually emphasize equiatomic compositions, which restrict the compositions available to induce strengthening via precipitation. Here the authors use spinodal decomposition in a five-element alloy to obtain high content nanophases and the highest tensile strength reported to date.

Study on Preparation and Corrosion Resistance of Graphene-Ni 35 Co 30 Cu 20 Fe 15 High Entropy Alloy Composites

The graphene/Ni 35 Co 30 Cu 20 Fe 15 high entropy alloy composites were successfully prepared by ball milling and spark plasma sintering (SPS) methods. The microstructures and structures of graphene and composites were measured by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The hardness and corrosion resistance of the composites were tested by Vivtorinox hardness tester and chemical workstation. The results showed that the homogeneous distribution of graphene in the composites can be achieved by ball milling, and the original structure of graphene was not destroyed. The addition of graphene improved the hardness and corrosion resistance of high entropy alloy composites. Compared with the Ni 35 Co 30 Cu 20 Fe 15 high entropy alloy, the hardness of the high entropy alloy with 0.3 wt.% Graphene increased from 255HV to 310HV. The addition of graphene increased the corrosion potential of the composite in 3.5% NaCl solution from -0.5V to -0.2V and the corrosion current density from 2×10 -5 A/cm 2 reduced to 8×10 -6 A/cm 2 , which greatly improved the corrosion resistance of the material.